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Business Concentration (Details) - Geographic Concentration Risk
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
North Carolina      
Concentration Risk [Line Items]      
Concentration percentage 40.30% 37.80% 35.00%
Texas      
Concentration Risk [Line Items]      
Concentration percentage 18.50% 18.90% 19.70%
South Carolina      
Concentration Risk [Line Items]      
Concentration percentage 10.20% 10.50% 10.30%